The present invention relates to alloys and more particularly to wrought nickel-based alloys which are useful for fabricating components of a high-temperature gas-cooled reactor.
The high-temperature gas-cooled reactor (HTGR) is a graphite-moderated, helium-cooled system capable of providing helium at temperatures as high as 850.degree. C. to 1050.degree. C. This helium can be used to heat steam to drive a turbine, as in a steam cycle plant, or to be used directly as in a gas-turbine power plant. More recently, emphasis has shifted to process heat applications, making the HTGR useful for a wide variety of needs such as steel or synthetic fuel production. It is in this latter capacity where the advantages of a high-temperature nuclear system are exploited to their full advantage.
As operating temperatures are increased to take advantage of efficiency considerations as well as to promote the utility of the systems, the materials requirements become more and more difficult to satisfy. Many of the metallic components are required to withstand temperatures in the range of 850.degree. C. to 1050.degree. C. for the lifetime of a reactor. Process heat applications, such as coal gasification, accentuate the need for higher temperatures and stringent materials capabilities. These capabilities include strength and corrosion resistance at temperatures where ordinary alloys are limited. For applications in which materials may be subjected to irradiation in a thermal neutron field, e.g. within the HTGR core containment cavity, additional capabilities may be required to limit production of transmutation species which can affect the mechanical integrity of the materials or operating efficiency of the system.
Corrosion may occur in nuclear reactors as a result of oxidation and carburization, among other processes, depending upon the alloy chemistry, the coolant composition and the internal reactor temperature. Carburization has been identified as the key concern for metallic structural materials in a HTGR. Attendant with the increase in carbon concentration in a carburized alloy is an increase in carbide precipitation particularly along planar defects, such as grain and twin boundaries. The primary effect of this additional carbide precipitation has been found to be a decrease, sometimes substantial, in tensile and creep ductilities. In some cases, a decrease in creep rupture lifetimes has been observed.
Carburization, as well as other corrosion processes, occur in a HTGR partly due to the practically inevitable impurities in the helium coolant. The impurities usually include hydrogen, methane and carbon monoxide under partial pressures as high as 5.times.10.sup.-4 atm. Water is sometimes present, but in much lower concentrations. These impurities infuse into and interact with metallic components contributing to their deterioration.
Candidate materials for HTGRs have been evaluated using simulated reactor helium environments, e.g. 0.9995 atm. of helium, 5.times.10.sup.-4 atm. of hydrogen, 5.times.10.sup.-5 atm. of methane, 5.times.10.sup.-5 atm. of carbon monoxide and trace amounts of water at temperatures ranging from 800.degree. C. to 1000.degree. C. A number of alloys have been tested for high-temperature strength and resistance to carburization. By way of example, IN100 (nominal composition: 50% Ni, 15% Co, 10% Cr, 5.5% Al, 4.7% Ti, 3.0% Mo, 0.18% C, 0.014% B, 0.06% Zr, and 1.0% V) and IN713LC (nominal composition: 75% Ni, 12% Cr, 4.5% Mo, 2.0% Nb, 0.05% C, 5.9% Al, 0.6% Ti, 0.10% B and 0.10% Zr) have proved to exhibit excellent high-temperature strength, with the former also having excellent resistance to carburization. (All concentrations are by weight, unless otherwise indicated.) An experimental alloy (nominal composition: 10.6% W, 6.06% Cr, 4.76% Al, 3.25% Ti, 2.05% Mo, 1.43% Nb, 0.11% Zr, 0.108% C, 0.028% B, less than 0.05% Si, less than 0.05% Mn, and the balance Ni) formed by adding 3.25% titanium to alloy M21 was found to exhibit excellent carburization resistance at high temperatures. (Ennis, P. J. "Investigations of Experimental and Modified Commerical Alloys for the Project PNP, KFA-IRW-TN-132/78", November 1978.) These alloys may be cast to shape and thus are suitable for HTGR components which can be cast to shape, such as turbine blades and vanes, and thermal barrier covers. However, none of these metals are known to be workable, e.g. cold workable or hot workable. Thus, HTGR components which require fabrication must be formed from other alloys.
No commercially available wrought alloy is known to be suitable for fabricating components for HTGR environments. Hastalloy X (nominal composition: 22% Cr, 9% Mo, 1.5% Co, 0.5% W, 18.5% Fe, and the balance Ni) and Inconel 617 (nominal composition: 22% Cr, 9% Mo, 12.5% Co, 1% Al, and the balance Ni) are workable, but exhibit unsatisfactory resistance to carburization. Additionally, a material is needed which is adapted for use in a region of high thermal neutron flux.